The present invention relates to the fabrication of semiconductor materials and, more particularly, to devices for use in fabricating silicon carbide crystals and the fabrication of silicon carbide crystals, for example, to produce silicon carbide boules from which a silicon carbide wafer may be provided.
Silicon carbide (SiC) is rarely found in nature. It has, however, been manufactured for more than eighty years, in crystalline form, for abrasive products. Silicon carbide crystals found in nature and in abrasive products are generally black and not translucent because they contain substantial levels of impurity atoms.
In the 1950""s the Lely process was developed at General Electric Company by which silicon carbide was sublimed and randomly deposited to produce small, thin silicon carbide crystals that were used in early silicon carbide semiconductor device development.
Because of the theoretically quite favorable electronic properties of silicon carbide, significant development activities were initiated during the 1960""s and 1970""s with the objective of growing large (bulk) crystals of low impurity silicon carbide for use in the production of semiconductor devices. These efforts finally resulted in the commercial availability of relatively low impurity, translucent silicon carbide crystals.
North Carolina State University was issued U.S. Pat. No. 4,866,005 and Re. 34,861 on the growth of single crystal silicon carbide through a sublimation process. The disclosure of U.S. Pat. No. 4,866,005 and Re. 34,861 are incorporated herein by reference as if set forth fully herein. Since that time, the growth of silicon carbide crystals has been described in several United States Patents, including commonly assigned U.S. Pat. No. 6,045,613 entitled xe2x80x9cPRODUCTION OF BULK SINGLE CRYSTALS OF SILICON CARBIDE,xe2x80x9d the disclosure of which is incorporated herein by reference as if set forth fully herein. Additional patents relating to the growth of silicon carbide or silicon carbide alloys include commonly assigned U.S. Pat. No. 6,048,813 entitled xe2x80x9cSIMULATED DIAMOND GEMSTONES FORMED OF ALUMINUM NITRIDE AND ALUMINUM NITRIDE: SILICON CARBIDE ALLOYS,xe2x80x9d and U.S. Pat. No. 6,086,672 entitled xe2x80x9cGROWTH OF BULK SINGLE CRYSTALS OF ALUMINUM NITRIDE: SILICON CARBIDE ALLOYS,xe2x80x9d the disclosures of which are incorporated herein by reference as if set forth fully herein.
One difficulty with silicon carbide is that silicon carbide may contain micropipes or other defects, such as dislocation defects. Such defects may reduce the suitability of regions of a silicon carbide wafer containing such defects for use in a semiconductor device. For example, a transistor with such defects incorporated therein may have a higher leakage current than a corresponding transistor without such defects. Accordingly, improvements may be needed in the growth of silicon carbide crystals.
In gallium arsenide (GaAs) and gallium nitride (GaN) growth through chemical vapor deposition (CVD), epitaxial lateral overgrowth (ELOG) and pendeo-epitaxial growth techniques have been utilized to reduce defects in layers of GaAs or GaN. Such techniques are, for example, illustrated in U.S. Pat. No. 4,522,661. U.S. Pat. Nos. 6,051,849 and 6,177,688. However, growth of silicon carbide crystals (e.g. boules) suitable for providing wafers or substrates is typically not carried out using CVD processes but is performed using physical vapor transport (PVT) growth, such as through a sublimation growth process as described above.
Embodiments of the present invention provide methods for producing silicon carbide crystals, seed crystal holders and seed crystals for use in producing silicon carbide crystals and silicon carbide crystals. According to embodiments of methods according to the present invention, silicon carbide crystals are produced by forcing nucleation sites of a silicon carbide seed crystal to a predefined pattern (e.g. nonrandom) and growing silicon carbide utilizing physical vapor transport (PVT) so as to provide selective preferential growth of silicon carbide corresponding to the predefined pattern.
In further embodiments of the present invention, nucleation sites are forced to the predefined pattern by modulating a thermal profile of the seed crystal. Such a modulation may be provided by forming regions of higher thermal conductivity in a seed crystal holder. The regions of higher thermal conductivity correspond to the predefined pattern. The seed crystal is mounted on the seed crystal holder.
The regions of higher thermal conductivity may be formed by removing portions of the seed crystal holder so that the seed crystal selectively contacts the seed crystal holder. The regions of higher thermal conductivity may correspond to regions where the seed crystal contacts the seed crystal when the seed crystal is mounted on the seed crystal holder.
Alternatively, the regions of higher thermal conductivity may be formed by removing portions of the seed crystal holder so as to provide cavities in the seed crystal holder and filling the cavities in the seed crystal holder with a material having a higher thermal conductivity than a material of the seed crystal holder. Such cavities may be filled by covering the seed crystal holder with a layer of the material having a higher thermal conductivity and removing a sufficient amount of the layer of the material of higher thermal conductivity so as to expose portions of the seed crystal holder. In particular embodiments of the present invention, the material of the seed crystal holder is graphite and the material of higher thermal conductivity is silicon carbide.
In further embodiments of the present invention, the predefined pattern may be stripes in the seed crystal holder. The predefined pattern could also be a pattern of posts in the seed crystal holder. The posts may be substantially circular.
In additional embodiments of the present invention, the nucleation sites may be forced to a predefined pattern by forming a pattern on an exposed surface of the seed crystal so as to provide regions of the seed crystal which extend beyond other regions of the seed crystal. Again, the pattern may include stripes in the seed crystal, a plurality of posts, and/or a plurality of substantially circular posts.
In still further embodiments of the present invention, nucleation sites are forced to the predefined pattern by forming a pattern of material other than silicon carbide on the silicon carbide seed crystal so as to provide a pattern of regions of having a reduced sticking coefficient over other regions of the seed crystal. Possible patterns may include stripes on the seed crystal, a plurality of posts on the seed crystal and/or a layer of material having a plurality of opening therein so as to expose potions of the seed crystal. The opening in the layer may be substantially circular. Furthermore, the material other than silicon carbide may be graphite.
In yet additional embodiments of the present invention, a seed crystal holder for growing silicon carbide using physical vapor transport is provided. The seed crystal holder has a body section configured to hold a silicon carbide seed crystal and a plurality of regions of differing thermal conductivity in the graphite body section. The regions of differing thermal conductivity have a predefined pattern and, along with the body section, are configured to produce a thermal profile in the seed crystal corresponding to the predefined pattern.
The plurality of regions may be a plurality of cavities in the body section. Alternatively, the plurality of regions of differing thermal conductivity may be configured to contact the seed crystal. In such embodiments, the regions of differing thermal conductivity may be regions of a material having a different thermal conductivity than the body section provided within the body section. The regions of differing thermal conductivity may have a lower thermal conductivity than the body section of the seed holder or a higher thermal conductivity than the body section of the seed holder. In particular embodiments of the present invention, the body section is made of graphite and the regions of differing thermal conductivity are regions of silicon carbide in the body section. Additionally, the predefined pattern comprise stripes in the seed crystal holder, a pattern of posts in the body section and/or a pattern of substantially circular posts.
In still further embodiments of the present invention, a silicon carbide seed crystal is provided. The silicon carbide seed crystal has a first surface and a second surface, opposite the first surface and a predefined pattern formed in the second surface so as to provide a plurality of regions of silicon carbide which extend a first distance form the first surface of the silicon carbide crystal and a plurality of other regions which extend a second distance, different from the first distance, from the first surface of the silicon carbide crystal. The pattern is preferably formed so as to preferentially grow silicon carbide in regions corresponding to the predefined pattern utilizing physical vapor transport growth. Furthermore, the first distance may be greater than the second distance. In such a case, the regions corresponding to the pattern may correspond to sidewalls of the regions of silicon carbide which extend the first distance from the first surface. The pattern may be stripes of trenches in the seed crystal, a plurality of posts and/or a plurality of substantially circular posts.
In additional embodiments of the present invention, a silicon carbide seed crystal is provided including a silicon carbide crystal having a first surface and a second surface, opposite the first surface and regions of a material other than silicon carbide on the second surface of the silicon carbide crystal. The material other than silicon carbide has a sticking coefficient associated with vapor phase transport growth of silicon carbide which is less than a sticking coefficient of silicon carbide. The pattern may be stripes of the material other than silicon carbide on the silicon carbide crystal, a plurality of posts, a plurality of substantially circular posts and/or a layer of the material other than silicon carbide having a plurality of openings to expose portions of the silicon carbide crystal. The openings may be substantially circular.
In further embodiments of the present invention, a silicon carbide crystal is provided having first regions of silicon carbide having a first defect density and second regions of silicon carbide having a second defect density, the second defect density being less that the first defect density. The first and second regions form a predefined pattern. The predefined pattern may be a pattern of stripes and/or a pattern of circles. The second defect density may be about five times less than the first defect density and, preferably, is about 10 times or more less than the first defect density.